If you are looking at being to big to go through doors, the issue is the appearance animation size, not the creature size. While the game does allow for resizing of appearances, it uses an invisible appearance with a "tail" for the actual appearance to accomplish this. Since polymorph.2da only supports the appearance change, you would not only have to change the appearance to be one of the resizing ones, but you would also have to change the script to add a tail, and then remove the tail when the polymorph is cancelled.
To make the use of the dragon shape feat infinite times per day, you would just need to edit the dragon shape line in feats.2da.